Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials. The junction of the n-type layer and the p-type absorber layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type absorber layer (i.e., an electron accepting layer) and the CdS layer acts as an n-type layer (i.e., an electron donating layer).
A transparent conductive oxide (“TCO”) layer is commonly used between the window glass and the junction forming layers. This TCO layer provides the front electrical contact on one side of the device and is used to collect and carry the electrical charge produced by the cell. Conversely, a back contact layer is provided on the opposite side of the junction forming layers and is used as the opposite contact of the cell. This back contact layer is adjacent to the p-type absorber layer, such as the cadmium telluride layer in a CdTe PV device.
However, metal electrodes can make a poor contact with the p-type layer, especially when the p-type layer includes cadmium telluride. This contact problem can lead to significantly reduced energy conversion efficiency in the device, and can lead to an increased rate of deterioration of the device.
Thus, a need exists for photovoltaic devices having improved energy conversion efficiency and/or device lifetime through improved contact between the back contact and the p-type layer, particularly when the p-type layer includes cadmium telluride.